What's the Difference Between a Leader and a Fly Line?

Gummy, rubbery fly line is heavy on the business end of a fishing rod. A real, thin monofilament kind of thing tapers all the way down to almost nothing. And that's what you tie on to the fly. You don't catch the fish with the fly line. You catch the fish on the end of your leader. The clear, tiny, little, almost invisible, tapered line.